1. Field of the Invention
The present invention relates to a reticle gripper to hold a reticle of a lithographic apparatus, a lithographic apparatus including such reticle gripper, a reticle handler robot including such reticle gripper, a device manufacturing method, and a method to bring a reticle to a reticle support.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, it is common that a reticle (also referred to in this document as patterning device or mask, the term reticle may in this document be understood to be a synonym for pattering device and/or mask), is exchanged from time to time. This may be to use the lithographic apparatus to apply a different pattern onto a substrate, e.g. a different layer of a semiconductor device, or to apply different patterns onto e.g. different parts of a surface of a substrate. The reticle may also be exchanged for other reasons. When exchanging the reticle, the reticle may be displaced by a gripper, which is constructed to hold the reticle. The gripper may form part of a reticle handler robot, and may e.g. be included in or connected to an arm of such robot. Requirements on the reticle gripper are high. This is because it is desirable that the reticle be handled carefully to avoid damage or contamination. In particular, the transfer of the reticle to a reticle stage, the reticle stage to hold the reticle, is important because any deformation of the reticle might impair the performance of the lithographic apparatus.
In order to avoid a deformation of the reticle, existing gripper designs have made use of elastic averaging to hold the reticle to the gripper. Elastic averaging relies on a (forced) geometric congruence between the gripper and the reticle. Further, for the reticle gripper, vacuum pressure is used to force the reticle into contact with the gripper. The gripper is in such existing designs provided with nominally flat interface surfaces, to avoid as much as possible a deformation of the reticle. Such gripper design to hold the reticle can however create problems as the forced geometric congruence may lead to undesired deformations of e.g. the reticle. Manufacturing limitations result in relatively large non-flatness errors in the contacting surfaces (thus e.g. the interface surfaces of the gripper and/or the surface of the reticle). As a result thereof, the reticle may be warped or bowed when held by the gripper. In part, such deformations of the reticle may recover because of an elasticity of the reticle, however some or all of these unwanted deformations of the reticle may continue to exist when the reticle is in use in the lithographic apparatus.
Furthermore, in present a lithographic apparatus, the transfer from the gripper to the reticle stage takes place such that for a brief period the reticle is held by vacuum pressure to both the gripper and to the reticle stage. If the reticle is deformed by the gripper, some of the deformation may have no opportunity to recover because of the mechanics of the transfer with simultaneous holding by gripper and reticle stage. In other words, the transfer may “lock in” the deformed shape of the reticle.
In general, deformations of the reticle may result in overlay errors in the photolithographic process performed by the lithographic apparatus.